Glossary of Astronomical Terms

Altitude
The angle above the horizon of a particular object. Also known as Elevation.

Azimuth
The angle from north of a particular object.

Constellation
A convenient grouping of stars found in the same area of the night sky. Today astronomers recognise 88 official constellations, most of which were created by early Greek, Egyptian and Babylonian astronomers. A constellation is a suburb of the night sky.

Elevation
See Altitude.

Galaxy
A galaxy is an island of stars in moving in space. Our galaxy is called the Milky Way. Galaxies come in different shapes, the most common being spiral and elliptical. The Milky Way is a spiral galaxy

Light-year
The distance light travels in one year - used as a unit of measuring distance for astronomical objects. Approximately 10 million million kilometres.

Meridian
A reference line which goes from north to south and passes through the Zenith.

Transit
The point in time when an object crosses the Meridian. For observers in the southern hemisphere the object will then be directly north and at its highest in the sky (at maximum Elevation).

Twilight
The time before sunrise and after sunset when the Sun is not in the sky but there is still some useable light. 

There are three levels of twilight that occur both at night and in the morning:  Civil, Nautical and Astronomical, and are all defined by the amount the centre of the Sun is below the horizon. Civil is 6°, Nautical is 12° and Astronomical is 18° below an ideal horizon. In the morning, it is the beginning of the twilight, and in the evening it is the ending of the twilight. 

Until the end of civil twilight, you can, in the absence of moonlight and artificial light, still see objects clearly enough to be able to get about. At this time, most people can still read outside.

Until the end of nautical twilight, you can, in the absence of moonlight and artificial light, still clearly discern the horizon. This is important for navigational sightings of a star because the use of a sextant requires you to sight the star and the horizon simultaneously.

Until the end of astronomical twilight, the Sun is still adding some glow to the sky, so astronomers wanting a truly dark sky need to wait until after this time to open their telescopes.

Zenith
The point in the sky directly overhead.

Comments (7)

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zahra 26 July, 2009 13:57
hello, it was great information.
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Cody 13 November, 2010 03:44
great info. might want to include in the twilight definition the differences between astronomical twilight, nautical twilight, and civil twilight.
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Discovery Centre 3 December, 2010 16:48
Hi Cody, thanks for your feedback, we've added some information to the infosheet about twilight!
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shae 22 December, 2010 13:35
good info
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shae 22 December, 2010 13:38
include some info about earth
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Discovery Centre 23 December, 2010 11:05

Hi Shae,

We have an infosheet all about the Earth here.

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Oscar 28 May, 2014 04:46
Hey... great work. but i was wondering, wat u add some interesting pictures and animations to define the terms more properly...
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